Disable Prorate Across All Item Lines for New Freight Lines for Inventory PO Matched AP Invoices

Hi All,

How can we disable Prorate Across All Item Lines when New Freight Lines are being added to AP Invoices when the invoice is for a Inventory PO ( not expense).

We want this option to be non-editable for new Freight lines for such cases.
This is needed because currently 'Prorate Across All Item Lines' option is prorating to the account mapped to the PO line item. We don't want this. We want the Freight line to go to a fixed Freight account always. Is there a way to have the distribution account be a fixed Freight account if user selects Prorate Across All Item Lines? We don't want this specified in the Payables Options - as the Distribution Freight Account for Expense PO Invoices needs to be a different account.